Free-agent guard Jason Terry is close to agreeing to a three-year contract with the Boston Celtics that would pay him the full mid-level exception of about $5 million per year. The deal was reported Tuesday by ESPN Boston. Sources also told ESPN Dallas that the Mavericks are unlikely to match the Celtics' offer. Terry, 34, spent the past eight seasons in Dallas, which included winning the Sixth Man of the Year Award in 2009 helping the Mavericks win the NBA title in 2011. He averaged 15.1 points per game last season. Dallas is still waiting for free agent point Deron Williams to decide whether he will return to his hometown Mavericks or re-sign with the Brooklyn Nets.